The present invention relates to a cryostat for cooling and holding a specimen with an extremely low temperature liquefied gas such as liquefied helium to carry out various cryogenic experiments and measurements of such specimens as semiconductor materials, metallic materials or various kinds of elements, particularly a method for bonding a cable to a slit and sealing the slit in a work for passing the cable for the specimen through the inner wall of the vessel in the cryostat which has the inner wall made of fiber reinforced plastics (FRP).
Lately, along with an advancement of cryogenic science, the opportunities of cryogenic measurements and experiments are increasing to investigate cryogenic characteristics and behaviors of various materials and elements for semiconductors and other devices. In the cryogenic testing equipment for these measurements and experiments, specimens are usually cooled and held at a specified low temperature with a low temperature liquefied gas called cryogenic liquid such as liquefied helium or nitrogen, and such cooling equipment is generally referred to as the "cryostat".
This cryostat is primarily classified into an immersion type in which the specimen is directly immersed for cooling into cryogenic liquid and a thermal conduction type in which the specimen is indirectly cooled by thermal conduction without immersing into cryogenic liquid. In case of the latter thermal conduction type cryostat which employs the indirect cooling method, it is often difficult to fully cool the specimen to a cryogenic liquid temperature due to heat-in-leak and heat generation from the specimen. Therefore, the former immersion type cryostat is more advantageous in view of the temperature margin.
An example of the conventionally typical immersion type cryostat is shown in FIG. 4.
In FIG. 4, the cooling vessel 2 having the cryogenic liquid vessel 1 has a double-wall construction made up of the inner wall 3 and the outer wall 4 with the vacuum thermal insulation space 5 interposed therebetween. The specimen 6 is held at the extreme end of the pipe-shaped support member 8 extended from the top flange 7 and immersed into the cryogenic liquid 9 contained in the vessel 1. The signal line 10 for transmitting the signals between the specimen 6 and an electronic circuit of equipment (not shown) is guided from the specimen 6 to the top flange 7 through the inside of the support member 8 and out of the cryostat.
In such conventional immersion type cryostat, since the specimen 6 is suspended from the top flange 7 and immersed in the cryogenic liquid 9, the signal line is led out through the support member 8 for suspending the specimen as described above and therefore the length of the signal line 10 is larger than the depth of the cryogenic liquid vessel 1 of the typical cryostat which is usually longer than one meter. If the signal line is long as described above, the transmission of a signal from the specimen to the electronic circuit of the external equipment may be delayed for a time proportioned to the length of the signal line and a system using high speed devices will not function satisfactorily. For example, if an experiment or a measurement of the Josephson device which has an important feature of high speed operation and has lately been noted, is conducted with the conventional immersion type cryostat, there will be a problem that the transmission of signals will be delayed due to a long signal line and the high speed response of the whole system will therefore deteriorate.
A method to shorten the signal line from the specimen in the immersion type cryostat is to lead the signal line from the specimen to the outside across the vacuum thermal insulation space. The present inventors have already proposed the cryostat disclosed in the Utility Model Application KOKAI No. 3-564 as one of the immersion type cryostat having the above construction.
The cryostat proposed as above is characterized in that it basically comprises the upper vessel which is sealed with the top flange at its upper end and open at its bottom and the lower vessel which is remountably fitted to the lower part of the upper vessel to form, in conjunction with the upper vessel, the cryogenic liquid vessel. The upper vessel and the lower vessel have, respectively, a double-wall construction consisting of the inner and outer walls, the vacuum thermal insulation space is independently provided between the inner wall and the outer wall of the upper vessel and between the inner wall and the outer wall of the lower vessel. The lower vessel is provided with the specimen holding part for holding the specimen which is kept exposed in the cryogenic liquid vessel.
A practical example of the above proposed cryostat is shown in FIG. 5.
In FIG. 5, the upper vessel 20, which is formed to be hollow and cylindrical as a whole, is made to have a double-wall construction with the inner wall 21 and the outer wall 22 and provided with the vacuum thermal insulation space 23 between the inner wall 21 and the outer wall 22. The top flange 24 is remountably fitted with a bolt 25 to the upper end of the upper vessel 20 and the clearance between the top flange 24 and the upper surface of the upper vessel 20 is sealed with a sealing member 26 such as an O-ring. The top flange 24 is provided with the inlet port 28 for supplying cryogenic liquid 9 such as liquefied helium and the outlet port 29 for discharging a vaporized gas. The bottom of the upper vessel 20 is made open. The flange 30 is formed integral with the external periphery of the upper vessel 20 at a position as high as the specified distance l from the lower end.
On the other hand, the lower vessel 31 has a double-wall construction formed with the inner wall 32 and the outer wall 33 and is provided with the vacuum thermal insulation space 34 between the inner wall 32 and the outer wall 33. This lower vessel 31 comprises a large-diameter cylindrical part 31A which surrounds the lower part of the upper vessel 20, that is, the part corresponding to the distance l below the flange 30 and the rectangular parallel-piped part 31B which is integrally continued to the lower part of the large-diameter cylindrical part 31A and the bottom of the rectangular parallel-piped part 31B is closed. A pedestal type specimen holder 35 is formed on the internal bottom surface of the rectangular parallel-piped 31B. The upper end surface of the lower vessel 31 and the flange 30 of the upper vessel 20 are jointed with bolts 36 and the clearance between the upper surface of the lower vessel 31 and the flange 30 of the upper vessel 20 is sealed with the sealing member 37 such as the O-ring. The lower vessel 31 is supported by the base 38 and the support 39.
The cryogenic liquid vessel which stores the cryogenic liquid 9 such as liquefied helium is formed by the internal surfaces of the upper vessel 20 and the lower vessel 31 as described above. The specimen 6 is held on the specimen holder 35 and directly exposed to cryogenic liquid 9. The signal line 10 from the specimen 6 is led out of the rectangular parallel-piped part 31B of the lower vessel 31 through the inner wall 32, vacuum thermal insulation space 34 and outer wall 33 and connected to the external terminal 40 provided on the base 38.
In such an immersion type cryostat, the signal line 10 for transmitting and receiving the signals between the specimen 6 and the external equipment can be led out from the specimen holder 35 in the lower vessel 31 through the inner wall 32, vacuum thermal insulation space 34 and the outer wall 33 of the lower vessel 31. The delay in transmission of the signals depending on the length of the signal line 10 can be reduced by shortening the length of the signal line 10.
In the above proposed cryostat, the upper vessel 20 is separated from the lower vessel 31 when replacing the specimen. In this case, the vacuum thermal insulation space 23 of the upper vessel 20 and the vacuum thermal insulation space 34 of the lower vessel 31 are independent and therefore these vacuum thermal insulation spaces can maintain the vacuum condition. Accordingly, evacuation is not required after replacing the specimen and the working time can be substantially reduced. Since the vacuum thermal insulation space 23 of the upper vessel 20 and the vacuum thermal insulation space 34 of the lower vessel 31 are independent one from another, the vacuum sealing part is not required for the cryogenic position and the vacuum sealing work for the cryogenic position need not be carried out when bonding the upper vessel 20 and the lower vessel 31 after replacing the specimen.
In addition, the capacity of the cryogenic liquid vessel 1 can be increased by expanding the upper vessel 20. In case the upper vessel 20 is separated from the lower vessel 31, the setting and removal of the specimen 6 on and from the specimen holder 35 of the lower vessel 31 can be performed extremely easily by hand. Since the length of the upper vessel 20 has nothing to do with the operational efficiency in replacement of the specimen, the capacity of the crogenic liquid vessel 1 can be changed as required without deteriorating the operational efficiency in replacement of the specimen.
For the cryostats for use in measurements of magnetic characteristics, generally, non-magnetic materials are appropriate as constructional materials such as the inner and outer walls of the upper and lower vessels and a non-magnetic FRP has lately been often used as such non-magnetic materials. Glass fiber and epoxy resin are generally used as materials of the FRP. It is preferable to use, as the signal line (cable) for transmission of signals between the specimen inside the cryostat and external equipment, a flat tape type cable which is insulation-covered with a polyimide film (polyimide film cable) for cryogenic resistance as to mechanical characteristics and heat-in-leak through the cable. A method for passing and fixing such polyimide film cable through the inner and outer walls made of FRP material of the cryostat is usually, as shown in FIGS. 6 and 7, such that, for example, a slit 50 is formed in the inner wall 23 made of FRP, the polyimide film cable 51 is passed through this slit 50 and bonded to the internal surface of slit 50 with an epoxy adhesive 52 and simultaneously the slit 50 is sealed with this adhesive. However, this method has a problem as described below.
Specifically, the bonding area of the inner wall is inevitably small because of its thin thickness of approximately 3 mm in general and polyimide is a stable substance with inferior adhesiveness to other materials. Therefore, if a cryogenic liquid is transferred into the cryogenic liquid vessel of the cryostat and the bonded part (sealed part) of the polyimide film cable 51 and the slit 50 of inner wall 32 is cooled, the bonded part is prone to be cracked by a thermal stress produced. Particularly, an extremely large thermal stress takes place at the bonded part on the inner wall due to rapid cryogenic cooling and cracks as described above are apt to occur. Though the inner wall keeps the internal cryogenic liquid away from the external vacuum thermal insulation space, the cracks which have occurred in the bonded part of the polyimide film cable as described above will cause the cryogenic liquid to leak into the vacuum space and the liquid to vaporize, and the the vacuum of the thermal insulation space will deteriorate, thus rendering the cryostat unusable. For these reasons, in case of the cryostat made according to the prior art, the service life of its bonded and sealed part is extremely short and the cryostat can be used for operation only once or twice and therefore the cryostat has been disposed after each cryogenic operation.
From a further micro investigation as to the position where cracks have occurred in the bonded part of the polyimide film cable, it is clarified that cracks are not found in the boundary between the bond layer 52 and internal surface of slit 50 of the inner wall and all cracks were found in the boundary between the polyimide film cable 51 and the bond layer 52. From this fact, it is known that the bonding strength at the boundary between the internal surface of the slit of the FRP inner wall and the adhesive layer is sufficient but the bonding strength at the boundary between the polyimide film cable and the bond layer is insufficient when the polyimide film cable and the internal surface of the slit of the FRP inner wall are bonded and sealed with adhesive.
In case of the conventional method as previously described, air bubbles may be included in the adhesive when the polyimide film cable is inserted through the slit and the adhesive is applied or the adhesive may drool from the slit and the slit may not be fully filled with the adhesive. These problems have been a cause of insufficient bonding strength or a cause of gas leakage at an early stage. Though degassing or pressurization when applying the bond to the bonding part can be performed to prevent these problems, the cryostat itself has a large diameter and such degassing and pressurization have been actually impossible.